This invention relates to an air-fuel ratio feedback control system for controlling the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine by sensing and feeding back the oxygen concentration of engine exhaust gases which is representative of the air-fuel ratio of the supplied air-fuel mixture.
In order to purify exhaust gases from an internal combustion engine, it has heretofore been proposed to sense and feed back the oxygen concentration of exhaust gases in order to control the air-fuel ratio of an air-fuel mixture in accordance with the sensed value so that the ratio is equal to, for example, a constant theoretical air-fuel ratio. This method of feedback control has proved to be advantageous in that the air-fuel ratio remains constant even though major changes occur in outside environments such as atmospheric pressure variations, intake air temperature changes and so forth. Further, it has been verified that with the integration of the sensed air-fuel ratio value prior to feeding back improved control speed can be attained. However, if this feedback control is kept operative during all operating conditions of an internal combustion engine, a problem arises that desired power demands are not met at high engine speeds because the air-fuel ratio is held at a constant level. In addition, it has been proposed to enrich the air-fuel mixture by disabling the feedback control system at high loads in order to prevent abnormal increase in the reaction temperature due to the use of catalysts for purifying exhaust gases. However, from a fuel economy standpoint, it would not be advantageous to supply a rich mixture of a constant ratio while the feedback loop is disabled.